12.2 Energy Storage and Conversion

Energy storage technologies are crucial for modern power systems. From batteries to fuel cells, these systems convert and store energy in various forms, enabling efficient use of electricity and supporting renewable integration.

Thermodynamics governs energy conversion processes in storage systems. Understanding principles like Gibbs free energy and the Nernst equation helps optimize performance. Key factors include capacity, efficiency, and durability, which are essential for integrating renewables and creating a sustainable energy future.

Energy Storage Technologies

Principles of energy storage technologies

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• Batteries store electrochemical energy and convert it to electrical energy
  • Consist of anode, cathode, and electrolyte
  • Lead-acid batteries used in automotive applications and backup power systems
  • Lithium-ion batteries have high energy density, used in portable electronics (smartphones) and electric vehicles (Tesla)
  • Flow batteries are scalable, used in large-scale energy storage applications (utility-scale)
• Fuel Cells convert chemical energy from a fuel into electrical energy
  • Consist of anode, cathode, and electrolyte
  • Proton exchange membrane fuel cells (PEMFCs) operate at low temperature, used in transportation (fuel cell vehicles) and portable applications (backup power)
  • Solid oxide fuel cells (SOFCs) operate at high temperature, used in stationary power generation (combined heat and power systems)
• Supercapacitors store energy in an electric field between two electrodes
  • High power density and long cycle life
  • Used in regenerative braking in electric vehicles and power smoothing in renewable energy systems (wind turbines)

Thermodynamics of energy conversion

• Electrolysis splits water into hydrogen and oxygen using electrical energy
  • Gibbs free energy change $\Delta G$ determines minimum voltage required
  • $\Delta G = -nFE$, where $n$ is electrons transferred, $F$ is Faraday's constant, and $E$ is cell potential
  • Overpotential is additional voltage required to overcome activation and mass transport limitations
  • Tafel equation relates overpotential to current density, indicating reaction kinetics
• Fuel Cell Reactions convert chemical energy from a fuel into electrical energy
  • Nernst equation relates cell potential to concentrations of reactants and products
  • $E = E^0 - \frac{RT}{nF} \ln \frac{a_{products}}{a_{reactants}}$, where $E^0$ is standard cell potential, $R$ is gas constant, $T$ is temperature, and $a$ represents activity
  • Butler-Volmer equation describes relationship between current density and overpotential
  • Activation overpotential and mass transport overpotential affect fuel cell performance

Optimization of storage systems

• Capacity
  • Energy storage capacity is amount of energy that can be stored in a system
  • Power capacity is rate at which energy can be delivered or absorbed
  • Sizing energy storage systems based on application requirements (peak shaving, frequency regulation)
• Efficiency
  • Round-trip efficiency is ratio of energy output to energy input during a complete charge-discharge cycle
  • Coulombic efficiency is ratio of charge extracted from a system to charge input during a cycle
  • Minimizing energy losses through optimal design and material selection (advanced electrolytes, nanostructured electrodes)
• Durability
  • Cycle life is number of charge-discharge cycles a system can undergo before performance degrades below acceptable levels
  • Calendar life is expected lifespan of a system under normal operating conditions
  • Improving durability through advanced materials (solid-state electrolytes), thermal management, and control strategies (state of charge management)

Energy storage for renewable integration

• Renewable Energy Integration
  • Intermittency of renewable energy sources requires energy storage to balance supply and demand
  • Energy storage systems smooth power output fluctuations and shift energy delivery to peak demand periods
  • Types of energy storage for renewable integration:
    1. Pumped hydro storage: Large-scale, mature technology (Bath County Pumped Storage Station)
    2. Battery energy storage systems (BESS): Modular, fast-responding, and scalable (Hornsdale Power Reserve)
    3. Compressed air energy storage (CAES): Large-scale, long-duration storage (Huntorf CAES Plant)
• Sustainable Energy Future
  • Energy storage enables higher penetration of renewable energy sources, reducing reliance on fossil fuels
  • Decentralized energy storage systems improve grid resilience and support development of microgrids (community solar with storage)
  • Energy storage facilitates electrification of transportation, reducing greenhouse gas emissions (electric vehicle charging infrastructure)